A standard of a wireless local area network (WLAN) technology is being developed as a standard of Institute of Electrical and Electronics Engineers (IEEE) 802.11. IEEE 802.11a and IEEE 802.11b use an unlicensed band at 2.4 GHz or 5 GHz and IEEE 802.11b provides a transfer rate of 11 Mbps and IEEE 802.11a provides a transfer rate of 54 Mbps. IEEE 802.11g applies Orthogonal Frequency Division Multiplexing (OFDM) at 2.4 GHz to provide a transfer rate of 54 Mbps. IEEE 802.11n applies multiple input multiple output-OFDM (MIMO-OFDM) to provide a transfer rate of 300 Mbps for 4 spatial streams. IEEE 802.11n supports a channel bandwidth of up to 40 MHz. In this case, IEEE 802.11n provides a transfer rate of 600 Mbps.
IEEE 802.11af is a standard for defining operation of an unlicensed device in a TV whitespace (TVWS) band.
The TVWS band, which is a frequency band allocated to a broadcast TV, includes an Ultra High Frequency (UHF) band and a Very High Frequency (VHF) band. Specifically, the TVWS band is a frequency band which is allowed to be used by an unlicensed device under a condition that this does not hinder communication of a licensed device that operates in the frequency band. The licensed device may include a TV or a wireless microphone. The licensed device may be referred to as an incumbent user or a primary user. To overcome the problem of coexistence of unlicensed devices, a signaling protocol such as a common beacon frame, a frequency mechanism, or the like may be required for the unlicensed devices.
Operations of all unlicensed devices are permitted in a band of 512-608 MHz and a band of 614-698 MHz. However, only communication between fixed devices is permitted in bands of 54-60 MHz, 76-88 MHz, 174-216 MHz, and 470-512 MHz. The term “fixed device” refers to a device that performs signal transmission only at a fixed location. An IEEE 802.11 TVWS terminal is an unlicensed device that operates, in the TVWS spectrum, using IEEE 802.11 media access control (MAC) and physical (PHY) layers.
An unlicensed device that desires to use the TVWS band should provide a function to protect licensed devices. Accordingly, the unlicensed device should check whether or not a licensed device occupies the band before the unlicensed device starts signal transmission.
To accomplish this, the unlicensed device may check whether or not the band is being used by a licensed device by performing spectrum sensing. Examples of the spectrum sensing mechanism include an energy detection scheme and a feature detection scheme. The unlicensed device may determine that a licensed device is using a specific band when the intensity of a received signal is greater than a specific level or when a DTV preamble is detected. Upon determining that a licensed device is operating in a channel that is immediately adjacent to a channel currently used by the unlicensed device, the unlicensed device should reduce transmit power.
In addition, an unlicensed device should access a database (DB) through the Internet or a private network to acquire information of a list of channels that the unlicensed device can use in a corresponding region. The DB stores and manages information of licensed devices registered in the DB and channel usage information which dynamically changes according to the geo-location and usage time of the licensed devices.
In the description of this specification, the whitespace band may include, but need not be limited to, the TVWS described above. The term “whitespace band” in this specification refers to a band in which operation of a licensed device is preferentially allowed and operation of an unlicensed device is allowed only when licensed device protection is provided. The term “whitespace device” refers to a device that operates in the whitespace band. A device according to the IEEE 802.11 system may be an example of the whitespace device. In this case, the term “whitespace device” may refer to an unlicensed device that operates using 802.11 Medium Access Control (MAC) and Physical (PHY) layers in the whitespace band. That is, a general AP and/or STA according to the 802.11 standard which operates in the whitespace band may be an example of the unlicensed device.